The present invention relates to spectro photometers and, more specifically, to an accessory device for use with a spectro photometer to measure the optical characteristics of long, solid materials shaped like a film or plate.
Spectro photometers are employed to measured the spectral transmittance or absorbance ratio of liquid and solid materials at varying wavelengths. The chemical and other characteristics of the materials are inferred from the measured spectrum for qualitative or quantitative analysis of the materials.
A holding apparatus holds the sample or container in a stable position during analysis. Liquid samples are generally held in a standardized container such as, for example, a test tube. Thus, a single holder design suffices to hold most liquid specimens during measurement. Solid materials present a different problem since they occur in different shapes and sizes which must be accommodated by a holder for holding the samples in place in the spectro photometer.
Unlike liquid media, which can be transferred into any common liquid medium container and placed within the optical path of the spectro photometer for analysis, the need for an apparatus to hold solids has prompted the development of several sample holders of different shapes and sizes, including spheres, plates, and rods.
A particular problem, to which the present invention is addressed, is holding long thin, flexible materials such as photographic film or magnetic tape. Also of relevance to the present invention is apparatus for holding glassware used in reflective mirrors for photocopiers. These samples are analyzed by spectro photometers to insure that the chemicals which coat the samples are uniformly distributed. The optical characteristics of these materials are calculated as a function of the distance from an emitted beam as the sample is moved through the optical path of the incident beam. Due to the relatively small size of a typical spectro photometer, however, there is very little space available for a sample holder or its sample.
Prior art spectro photometers employ two parallel beams of light: a measuring beam and a reference beam. Conventionally, both beams are split from the same light source. The measuring beam passes through the sample whose optical properties are being measured while the reference beam passes through a reference sample with known chemical characteristics. After the measuring beam emerges from the sample, the beams are compared for changes in intensity and wavelength. The use of two beams in this manner is termed "double beam measurement". The presence of the reference beam permits cancelling out changes in the received measuring beam due to systematic changes in source brightness.
A difficulty emerges when using double beam measurement for analyzing long, flat, solid samples. The geometry of a conventional spectro photometer is such that, when performing measurements on, a long object such as, for example, a glass rod, the rod intercepts the reference beam, as well as the measuring beam. As a result of this problem, when measuring long solid objects, only the measuring beam can be used in the analysis. The reference beam is turned off, blocked or ignored. This so-called "single beam measurement" is not as accurate as double beam measurement, since the source brightness of light sources of spectro photometers are subject to an inherent drift with power supply voltage and lamp age which is normally compensated for by the reference beam.
When analyzing a flat sample of film, the flexibility of the material allows it to be fed through the spectro photometer on a bent path which avoids interference with the reference beam. In one prior-art device, the film is fed into the spectro photometer through an inlet above the optical bath of the measuring beam. A film guide directs the film through the optical path of the measuring beam and sends it on a skewed path to an outlet. The inlet and outlet are separated by an offset distance large enough to prevent the film from crossing the optical path of the measuring beam twice, yet small enough to keep the film out of the path of the reference beam. Since the reference beam is unaffected, double beam measurement can be used.
Achieving double beam measurement for film in this manner is not without its disadvantages. First, the presence of the equipment which feeds and winds the film above the sample chamber makes it difficult to access the chamber through a simple cover. Second, the curved shape of the film guide make it difficult and expensive to produce. Also, the area of the film measured by the spectro photometer is not ideal.
The above prior-art device also suffers from a decrease in signal-to-noise ratio. Before reaching the film, the measuring beam must pass through a beam diaphragm to increase the resolution of the spectro photometer measurements. The opening in the diaphragm is rectangular in shape, and the base must extend the width of the film. The base in the horizontal plane is thus much longer than the side in the vertical plane. The measuring beam, however, is rectangular in shape with the base in the horizontal plane much smaller than the side in the vertical plane. The result is two overlapping rectangles at right angles to each other wherein a major part of the measuring beam is blocked by the diaphragm, resulting in a corresponding degradation in signal-to-noise ratio.